1. Field of the Invention
The present invention relates to a method of manufacturing a membrane electrode assembly for use in a solid polymer fuel cell or the like.
2. Description of the Related Art
Known membrane electrode assemblies for use in solid polymer fuel cells or the like comprise a pair of positive and negative electrodes and a polymer electrolytic membrane sandwiched between the positive and negative electrodes.
When a reducing gas such as hydrogen, methanol, or the like is supplied to the negative electrode (fuel electrode) of a solid polymer fuel cell, the reducing gas generates protons due to the action of a catalyst contained in the negative electrode. The protons move through the polymer electrolytic membrane toward the positive electrode (oxygen electrode). When an oxidizing gas such as air, oxygen, or the like is supplied to the positive electrode (oxygen electrode) while the reducing gas is being supplied to the negative electrode (fuel electrode), the protons react with the oxidizing gas, generating water, due to the action of a catalyst contained in the positive electrode. As a result, an electric current can be extracted from a conductive wire connected between the positive and negative electrodes.
The solid polymer fuel cell has its electric power generating capability governed by the ease with which the protons generated on the negative electrode move toward the positive electrode. For quickly moving the protons toward the positive electrode, it is necessary for the positive electrode and the polymer electrolytic membrane, which serves as a partition membrane, to be well kept in contact with each other, and also for the polymer electrolytic membrane to have a low resistance in the membrane electrode assembly. Therefore, the polymer electrolytic membrane should desirably be as thin as possible.
The membrane electrode assembly is manufactured by forming a pair of membrane-like electrodes and a polymer electrolytic membrane, placing the polymer electrolytic membrane between the electrodes, and pressing them together with a hot press. However, since the polymer electrolytic membrane is hot-pressed while it is being softened with heat, if the polymer electrolytic membrane is thin, then it may possibly be damaged to cause a gas leakage or a short circuit between the electrodes. This problem aggravates itself if the electrodes have a low level of smoothness.
The hot press is problematic in that when pressed, the electrodes become less permeable to air because they are compressed into a denser structure. Another problem of the hot press is that good contact cannot be achieved between the electrodes and the polymer electrolytic membrane.
To solve the above problems, there has been proposed a process of integrally forming a polymer electrolytic membrane and electrodes. According to the proposed process, a board such as a plastic film or the like is coated with an electrode dispersion containing a catalyst carried by a catalyst carrier and dispersed in a polymer electrolytic solution, and then dried into a membrane electrode. The membrane electrode is then coated with a polymer electrolytic solution, and then dried.
According to the proposed process, after two assemblies each comprising an electrode and a polymer electrolytic membrane which are integrally combined with each other are prepared, and the polymer electrolytic membranes of the respective assemblies are joined to each other, thus producing a membrane electrode assembly which comprises a polymer electrolytic membrane sandwiched between a pair of electrodes.
The polymer electrolytic membranes may be joined to each other by a process of superposing and hot-pressing the polymer electrolytic membranes, a process of superposing the polymer electrolytic membranes with a polymer electrolytic solution interposed therebetween and then drying the assembly, or a process of superposing the polymer electrolytic membranes as they are undried and then drying the polymer electrolytic membranes.
The above proposed process allows a polymer electrolytic membrane to be reduced in thickness, and can produce a membrane electrode assembly without causing damage to the polymer electrolytic membrane.
However, the proposed process is disadvantageous in that inasmuch as a membrane-like electrode is coated with a polymer electrolytic solution to produce a polymer electrolytic membrane, the applied polymer electrolytic solution tends to permeate the electrode, reducing the air-permeability of the electrode.